PROTEIDS. 411 



tissues, muscles, nerves, glands, and all other organs ; they are also 

 found in small quantities in nearly every part of plants, and in larger 

 quantities in many seeds. They have never yet been formed by arti- 

 ficial means, but are almost exclusively products of vegetable life, and 

 undergo but little change when consumed as food and assimilated by 

 animals. 



General properties. The various proteids resemble one another 

 closely in their properties. Their composition is so complex that, as 

 yet, no chemical formula has been assigned to them with any certainty, 

 The percentage composition and other reasons have led to a formula 

 represented by C 144 H 224 N 36 O 44 S 2 , which represents about the average 

 composition of the proteids. The percentage composition is shown in 

 the following figures : 



Carbon 50.0 per cent, to 55.0 per cent. 



Hydrogen . . . . . 6.7 " " 7.3 " 



Nitrogen 15.0 " 18.2 



Oxygen 20.0 " " 24.0 " 



Sulphur 0.3 "" 2.5 " 



Of other properties may be mentioned : 



1. They are amorphous, colorless, odorless, nearly tasteless sub- 

 stances, and, with the exception of peptones, incapable of dialysis. 



2. They are not volatile without decomposition. 



3. They easily undergo that decomposition known as putrefaction. 



4. Some are soluble in water, others only in water containing 

 alkalies, acids, or certain neutral salts, whilst some are insoluble. 



5. The soluble proteids are converted into insoluble modifications 

 either by heating to 60 or 70 C. (140 or 158 F.), or by the addi- 

 tion of mineral acids, alcohol, or certain metallic salts. This process 

 of converting soluble into insoluble proteids is called coagulation; 

 and proteids when once coagulated will not return to their original 

 soluble form without suffering some alteration. 



6. They are converted into peptones by the action of gastric juice. 

 (See later.) 



Analytical reactions. 



(Use a solution made by dissolving some of the white of an egg in about 

 10 parts of water, and filtering.) 



1. Proteids or their solutions are colored yellow by warm nitric 

 acid in consequence of the formation of a substance called xantho- 

 proteic acid. Addition of ammonia water changes the yellow color 

 to orange or red. 



